1. Technical Field
Embodiments disclosed herein generally relate to braze discs. More specifically, embodiments disclosed herein relate to brazing top loading cutters using braze discs.
2. Background Art
FIG. 1 shows one example of a conventional drilling system for drilling an earth formation. The drilling system includes a drilling rig 10 used to turn a drilling tool assembly 12 that extends downward into a well bore. The drilling tool assembly 12 includes a drilling string 16, and a bottomhole assembly (BHA) 18, which is attached to the distal end of the drill string 16. The “distal end” of the drill string is the end furthest from the drilling rig.
The drill string 16 includes several joints of drill pipe 16a connected end to end through tool joints 16b. The drill string 16 is used to transmit drilling fluid (through its hollow core) and to transmit rotational power from the drill rig 10 to the BHA 18. In some cases the drill string 16 further includes additional components such as subs, pup joints, etc.
The BHA 18 includes at least a drill bit 20, also known as a primary cutting structure. Typical BHA's may also include additional components attached between the drill string 16 and the drill bit 20. Examples of additional BHA components include drill collars, stabilizers, measurement-while-drilling (MWD) tools, logging-while-drilling (LWD) tools, subs, hole enlargement devices also known as secondary cutting structures (e.g., hole openers and reamers), jars, accelerators, thrusters, downhole motors, and rotary steerable systems.
In the field of downhole drilling, cutting elements commonly referred to as “cutters” are used to cut or shear the working surface of a well. Cutters are typically loaded into a cutter pocket formed in the cutting surface of a downhole tool and brazed within the cutter pocket.
Methods used to create cutter pockets and to affix cutting elements therein include numerous steps. For example, a cutter pocket may be machined into a cutting tool body using electric discharge machining (EDM) or laser machining. However, the machining process may scoop out a cutter pocket larger than needed. Thus, a welding process may be performed to create at least one shoulder in the scooped out cutter pocket on which a cutting element abuts. After the welding is complete and the cutting tool body and the welded shoulders are allowed to cool, the weld metal is cleaned of welding residue using a grinding process performed manually. This process may also be used to shape the weld shoulder and to remove any irregular surfaces. The imprecise nature of the manual shaping process may prevent manufacturers from achieving the tolerances indicated in the original cutter pocket design.
Alternatively, cutter pockets may be formed in a mold. For example, in forming cutter pockets in a matrix drill bit, cutting element place holders may be positioned into a mold of a drill bit. Matrix material may then be placed into the mold around the cutting element place holders. Once the bit is formed, the place holders are removed to reveal cutter pockets.
Next, cutting elements may be inserted into the cutter pockets so as to abut the weld shoulders or inner side walls of the cutter pocket. Cutters may be disposed on drilling tools according to several processes. Specifically, cutters may be disposed as either top loading cutters (meaning the cutting elements are disposed in cutter pockets from the top of a tool) or front loading cutters (meaning the cutting elements are disposed in cutter pockets from the front of a tool). Top loading cutters may be used in various downhole tools. Examples of such tools may include fixed cutter drill bits, also known in the art as drag bits or PDC bits, hybrid drill bits, milling devices, and mill-head thereof, hole opening devices, such as reamers, and other various tools knows to those of ordinary skill in the art. Similarly, front loading cutting elements may also be used in various downhole tools, such as drill bits, milling devices, and hole opening devices.
The cutting elements are typically fixed to the cutter pockets using a brazing process. In the brazing process, a braze material (sometimes referred to as filler material) is positioned between the cutting element and the cutter pocket. The braze material is melted and, upon subsequent solidification, bonds the cutting element in the cutter pocket. Brazing is a metal joining process that typically occurs at temperatures between about 1100° F. and 1800° F. In particular, brazing occurs at temperatures above the melting point of the braze material and below the melting point of the base materials being joined (e.g., the cutter pocket and the cutting element). Thus, selection of braze materials depends on their respective melting temperatures and on the materials being joined. Braze materials are generally defined as alloys having a melting temperature above 840 F, but less than the components to be joined. Braze materials may include aluminum, copper, nickel, silver, or gold based alloys, for example.
It is also common to conduct brazing processes at temperatures low enough to avoid damage to the material being joined. For example, when brazing cutting elements and/or cutter pockets having ultrahard material such as diamond, the brazing temperature is generally below 1400° F. and preferably below 1300° F. to avoid damage to the diamond material. This temperature restriction greatly limits the number of alloys that can be used as braze alloy for cutting elements with diamond layers thereon because most brazing alloys that provide sufficient shear strength for bonding cutting elements to a cutting tool also require brazing at temperatures above 1300° F. Therefore, alloys suitable for brazing cutting elements with diamond layers have been limited to only a couple of alloys which offer low enough brazing temperatures to avoid damage to the diamond layer and high enough braze strength to retain cutting elements on cutting tools. Further, in most manual brazing processes, it is difficult to control the brazing temperature.
Brazing may offer a higher joint strength than soldering (a lower temperature metal joining process, typically occurring at temperatures below 840° F.) while maintaining physical properties of the base material better than welding (a higher temperature metal joining process, typically occurring at temperatures above 1830° F.). During brazing, localized heating occurs which, consequently, requires cooling to take place. Due to differences in material properties, variable alignment, and dealignment of different material grains, cracks may initiate at the interfaces between the braze metal, the weld metal, and tool body.
For tools using front loading cutting elements, the cutting element may be brazed to a cutter pocket by first placing a braze alloy on the back surface of the cutter pocket, then placing the cutting element in the cutter pocket so that the braze is in between the cutting element and the cutter pocket, and finally applying heat to the assembly so that the braze material flows to the side surface. Such brazing methods may optionally include a rotating step, wherein the cutting element is rotated within the cutter pocket during the heating step.
Alternatively, a cutting element (either front loading or top loading) may first be placed in the cutter pocket and the braze material may then be placed in or near the cutter pocket such that it may flow from the top of the cutting element to the bottom of the cutting element, between the cutting element and cutter pocket, when it is heated to melting temperature. In brazing techniques where the braze material flows from the top to the bottom of a cutting element, voids may form throughout the braze, resulting in a weaker joint between the cutting element and the cutter pocket. Some brazing processes may also include rotating the cutting element during the heating step to try to prevent voids from forming in the braze between the cutting element and the cutter pocket improve braze coverage. Braze coverage refers to the amount of surface area to be brazed that contacts the braze material. For example, if the braze contacts the entire surface of the inner wall of the cutter pocket and the entire surface of the cutting element being brazed to the cutter pocket, the braze is said to have 100% braze coverage. In other words, a braze having 100% braze coverage has no pockets or voids within the cutting element/braze interface or the cutter pocket/braze interface.
Other typical brazing techniques for front loading cutting elements may include wrapping a braze foil or braze wire around the cutting element before loading it into the cutter pocket. Any excess braze material may protrude from the cutter pocket. Heat may then be applied to melt the braze material and join the cutting element to the cutter pocket. This method is also typically used for top loading cutting elements.
Although braze coverage has been improved in prior art methods for brazing front loading cutting elements, a brazing method for top loading cutting elements has not yet been developed that allows for improved braze coverage and braze thickness. For example, it is difficult to get the same results rotating top loading cutting elements as with front loaded cutting elements because a top loaded cutting element will have cutter pocket walls on all sides of the top loaded cutting element except for the top and cutting portion. Accordingly, there exists a need for improved brazing for top loading cutting elements.